Continuous web printing allows economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of paper or other print media material is fed past one or more printing subsystems that form images by applying one or more colorants onto the print media surface. With this type of printing system, finely controlled dots of ink are rapidly and accurately propelled from the printhead onto the surface of a moving print media, with the web of print media often coursing past the printhead at speeds measured in hundreds of feet per minute. During printing, variable amounts of ink may be applied to different portions of the rapidly moving print media web, with drying mechanisms typically employed after each printhead or bank of printheads. Variability in ink or other liquid amounts and types or variability in drying times can cause print media stiffness and tension characteristics to vary dynamically for different types of print media, contributing to the overall complexity of print media handling and print media dot registration.
U.S. Pat. No. 3,803,628, to VAN et al., entitled “Apparatus and method for positionally controlled document making,” discloses using a row of optical sensors to detect the location of the edge of the paper. The output of the sensor is used to control the placement of the printed image in the cross-track direction.
U.S. Pat. No. 3,913,719, to Frey et al., entitled “Alternate memory control for dot matrix late news device,” discloses the printing of cue marks on the paper by a rotary printing press. The start location for an inkjet printed image is measured out by counting encoder pulses following the detection of the cue marks.
U.S. Pat. No. 4,721,969 to Asano, entitled “Process of correcting for color misregistering in electrostatic color recording apparatus,” discloses printing of registration marks along each edge of the paper. The detected positions of these marks are used to adjust the placement of the subsequently printed image planes to account for offsets in the tracking of the paper and to account for elongation or shrinkage of the paper in the cross-track direction, and to account for skew of the paper as well.
Commonly-assigned U.S. Pat. No. 4,963,899, to Resch, entitled “Method and apparatus for image frame registration,” discloses an electrophotographic printer in which the in-track position of the web is monitored by detection of light passing through perforation in the web.
U.S. Pat. No. 5,093,674 to Storlie, entitled “Method and system for compensating for paper shrinkage and misalignment in electrophotographic color printing,” discloses a method for adjusting an image size for a channel of an electrophotographic printer by altering a scanning mirror speed.
U.S. Pat. No. 5,505,129 to Greb et al., entitled “Web width tracking,” discloses a method for tracking the width of a printed medium by detecting the edges of the medium.
U.S. Pat. No. 5,682,331 to Berlin et al., entitled “Motion tracking using applied thermal gradients,” and related U.S. Pat. No. 5,691,921 to Berlin et al., entitled “Thermal sensors arrays useful for motion tracking by thermal gradient detection,” provide a system using invisible thermal marks for tracking the motion of print media. A localized hot spot on the print media is formed by a thermal marking unit, and thermal sensor arrays downstream of the thermal marking unit in the system are used to detect the local hot spot. This approach is generally not compatible with printing systems in which dryers are located between thermal marking unit and the thermal sensor arrays because the heat provided by the dryers raises the background temperature, reducing the contrast of the thermal marks relative to the background. Furthermore, any non-uniformity in the heat profile provided by the dryer or air flow over the print media can produce non-uniform surface temperatures making it more difficult to detect the applied localized hot spot.
U.S. Pat. No. 6,068,362, to Dunand et al., entitled “Continuous multicolor ink jet press and synchronization process for this press,” discloses periodic printing of reference marks by a mark printer. Sensors upstream of subsequent printheads detect the reference marks. An encoder attached to the drive motor monitors paper motion. Variations in the detected spacings of the marks provides an indication of paper shrink or stretch. A pulse train is created in which the time between pulses is modified relative to the encoder pulse rate to account for the paper shrink and stretch. In some embodiments, the marks can fluorescent color marks printed on front or back side of the paper.
U.S. Pat. No. 6,362,847 to Pawley et al., entitled “Electronic control arrangement for a laser printer,” discloses a method for adjusting a length of a printed line by inserting or removing clock timing pulses.
U.S. Pat. No. 6,927,875 to Ueno et al., entitled “Printing system and printing method,” teaches a method for correcting for heat shrinkage by controlling a timing of light emission. The shrinkage is characterized by detecting media edges.
Commonly-assigned U.S. Pat. No. 8,123,326, Saettel et al., entitled “Calibration system for multi-printhead ink systems,” discloses a color-to-color registration system for a printer. Each of the printheads periodically prints registrations mark, and the registration marks are subsequently detected. Based on the detected relative position of the registration marks from the different color planes, corrections are made to bring the color planes into registration. In-track registration adjustments are made by frequency shifting the encoder pulse stream to account for shrink or stretch of the paper in the in-track direction. Because the registration corrections for a particular image plane are based on measured registration errors for one or more previously printed image planes, the corrections always lag behind the printing.
U.S. Patent Application Publication 2007/0172270 to Joergens et al., entitled “Method and device for correcting paper shrinkage during generation of a bitmap,” discloses a method for compensating for paper shrinkage by adding or removing image pixels, preferably in un-inked locations.
U.S. Patent Application Publication 2011/0102851 to Baeumler, entitled “Method, device and computer program to correct a registration error in a printing process that is due to deformation of the recording medium,” discloses a method for deforming an image to correct for registration errors, wherein the pixels to be deformed are selected stochastically.
European patent document EP0729846, to Piatt et al., entitled “Printed reference image compensation,” discloses the periodic printing of reference marks by an initial printhead. The reference marks are detected upstream of the printhead that overlays an image over the image printed by the first printhead. The reference marks are a collection of evenly spaced lines. The detected spacing of these lines at a downstream location, is used to identify paper stretch and shrink in the in-track direction. Data rates are adjusted to account for the detected paper shrink and stretch.
There remains a need for an improved system to track a position of a receiver medium as it travels along a media path.